1. Field of Invention
The present invention generally relates to a method of fabricating semiconductor devices, and more particularly to a method of determining a time to clean a low pressure chemical vapor deposition (LPCVD) system.
2. Description of Related Art
Chemical vapor deposition (CVD) is a widely-used thin film technology in fabricating semiconductor devices. The CVD method is typically performed in a reactor to solidify reactive gases into solid-state products to deposit a thin film on a wafer surface during a chemical reaction. Many thin films required in semiconductor devices are fabricated using the CVD method. For example, conductors, semiconductors, or dielectrics can typically be fabricated by using the CVD method. Also, the thin films are produced through a chemical reaction of gases by using the CVD method. The crystallinity, stoichiometry and other material characteristics of the thin films produced are therefore superior to those produced using a sputtering method. It is no wonder that the CVD method has become one of the most important technologies for deposition of thin films in fabricating semiconductor devices.
Basically, the CVD method can be categorized into two types depending on the operating pressure required by the CVD equipment, including an atmospheric pressure CVD (APCVD) and a low pressure CVD (LPCVD). The LPCVD generally has reactions undertaken at a lower pressure. The deposited thin films therefore have a better step coverage due to a slower depositing speed.
FIG. 1 shows a schematic block diagram of a conventional low pressure chemical vapor deposition (LPCVD) system, in which a reactor 100, a gas transporting system 102, an exhaust system 104, a process control system 106, and a vacuum pump 108 to provide a required vacuum level, are depicted. A deposition performed by using the LPCVD method generally requires a pressure of about 100 Torr or below, which is controlled by the vacuum pump 108. Reactive gases are pumped into the reactor 100 through the gas transporting system 102 to initiate a chemical reaction to deposit a solid-state product on a wafer surface. The pressure in the reactor 100 during a depositing process can be regulated by the vacuum pump 108 and a gas supplying source 110, so that the pressure in the reactor 100 can be maintained at a desired level. When the gas pressure in the reactor 100 is too low, the gas supplying source 110 fills the gap by supplying more gases into the reactor 100. On the other hand, when the gas pressure in the reactor 100 is too high, the excessive gases are pumped out through the exhaust system 104 by the vacuum pump 108. Furthermore, the processing gases in the reactor 100 after reaction are pumped out from the reactor 100. Solid deposition particles which are not deposited on the wafer surface accumulate in a particle filter/trap 112 during an exhausting process.
As the number of processing batches increases, the solid deposition particles not deposited on the wafer surface will gradually congest the particle filter/trap 112. When there is an insufficient gas pressure in the reactor 100, the gas supplying source 110 can not quickly provide the gases required to the reactor 100. On the other hand, when there is an excessive gas pressure in the reactor 100, the vacuum pump 108 can not smoothly pump out the gases out of the reactor 100. Because of the constraint of the LPCVD equipment, it is very difficult to accurately determine a time to clean the vacuum route, particularly, the particle filter/trap 112. When the thin films produced are too thick, which implies congestion in the vacuum route of the LPCVD system has occurred, it is too late to clean the vacuum system because the abnormal products need to be disposed of. Although this problem can be solved by increasing the cleaning frequency for the LPCVD system, it reduces, however, the throughput of the LPCVD system because idle time of the system is be increased.